This is a bit of a misinterpretation. It wasn't that the Ardennes were considered to be some kind of impassable barrier. It was believed, however, that they didn't have the kind of roads necessary to sustain a major military advance.

The Schleiffen Plan, Germany's basic war plan from the first World War, was at its heart a study of transportation. A particular number of men took up a particular length of road and took a particular time to traverse it; if they had multiple roads available, they could divide through those roads, but they still took up about the same amount of space. The total number of available roads was, in essence, the limiting factor for how many men could be moved through a given area in the minimum amount of time. (Of course troops could reach the border via train, but once they were in enemy territory, they had to move mostly on foot, with most of their supplies moving by wagon - at least for the initial offensive, when there wouldn't be time to construct additional roads or rail.)

The Ardennes didn't have the kind of road network through which one could move a large WW1-style army. There just wasn't ROOM to push them all through and keep them supplied.

But WW2 used different military logic. Motorized infantry divisions, and above all armored divisions, could move through an area much faster than men could walk on foot. That ultimately meant that more of them could go through the more-limited road network of the Ardennes. Further, once there, the armor was able to break through the local French defenses and continue into the rear in ways that a WW1-vintage army simply couldn't have accomplished.

The French had tanks, but they were largely split up between various divisions of infantry; they didn't have the kind of formations that the Germans had, and didn't have a good answer for them. The British, at the time, had virtually no tanks at all. Without experience of handling large armored formations, neither army had a particularly good idea of how to deal with the threat that they posed.

So to answer your question, it wasn't that the French hadn't studied the terrain (in fact, Churchill mentioned that there probably wasn't a stretch of terrain in the world that had received more attention than the French/German border areas) - it was that they viewed that terrain through the lens of the previous war, and made assumptions based on the capabilities of those forces rather than the capabilities that the Germans actually had.

Keegan has a good general history of the first World War, which covers the genesis of the Schlieffen Plan and its shortcomings in great detail; understanding the first war helps a lot when trying to figure out why people thought certain things at the beginning of the later war.

From an earlier answer of mine

The collective memory of French military leadership in 1940 has popularized images of myopia, overconfidence, and incompetence among the French high command. Commentators like William Shirer helped to popularize notions of a French leadership that was ill-suited for the type of warfare unleashed in 1940. However, things were not all that bad in the French Army and there were some realistic voices. The French conducted a map exercise in 1938 that actually war gamed a German thrust into the Ardennes and it presaged the Gelb thrust to an alarming degree.

The map exercises in question were conducted by General André-Gaston Prételat, the commander of Second Army between May-June 1938. As Julian Jackson describes in The Fall of France, Prételat's exercise estimated that it would take sixty hours for a tank-armed aggressor to cross the Ardennes. Nor was Prételat alone in his estimation of the Ardennes's suitability for tanks; a Colonel Bourguignon wrote in a study of frontier reconnaissance that the area was vulnerable to a surprise tank thrust.

The French high command was largely indifferent to these concerns. Gamelin accused Prételat of too much pessimism in his map exercise and felt that existing troop levels would be more than enough to staunch a German assault through the area. The provisions Petain staked defense on the region upon were special blocking troops of the French army and cooperation with the Belgians. Both of these would be sufficient to hold an unlikely German Ardennes offensive until the bulk of French forces could meet it and destroy the offensive in a set-piece, methodical battle. The French high command estimated that because of the roads and geography, a German attack would likely take nine days to clear the forest and two weeks to reach the Meuse. If a German attack were to materialize in this sector, then the French high command believed they would have ample time to organize an efficient counterattack. In addition to dismissing Prételat's exercise, Gamelin also restricted access to the results of the map exercise lest it create panic among the lower ranks over German capabilities.

Hindsight has rendered both Gamelin and Pétain rather foolish over this myopia over the Ardennes, and with a good deal of justification. Nonetheless, the German experience in the Ardennes does add some weight to the French military chiefs' skepticism of the Ardennes suitability for a quick thrust. Gelb created massive traffic jams in the area and the offensive did dance on the edge of disaster at a few critical points during the campaign because of road bottlenecks. French intelligence and aerial reconnaissance largely failed to pick up the build-up in the Ardennes and the French high command tended to dismiss those few reports that did indicate something was up in this sector. The French rigid system of command and control also denuded what little initiative the local blocking troops possessed to meet the upcoming onslaught. Additionally, Franco-Belgian cooperation in the Ardennes proved anything but unified with the Belgians evacuating areas with the false expectation French troops would take their places. This was in stark contrast to German troops who had a much more flexible approach to military operations and a unified command more attuned to the realities of the situation.

Sources

Citino, Robert Michael. Death of the Wehrmacht: The German Campaigns of 1942. Lawrence: University Press of Kansas, 2007.

Dildy, Doug, and Peter Dennis. Fall Gelb 1940 (1): Panzer Breakthrough in the West. Oxford: Osprey, 2014. 2014.

Epstein, Jonathan A. Belgium's Dilemma The Formation of the Belgian Defense Policy, 1932-1940. Leiden: Brill, 2014.

Frieser, Karl-Heinz, and John T. Greenwood. The Blitzkrieg legend: the 1940 campaign in the West. Annapolis, Md: Naval Institute Press, 2005.

Nord, Philip G. France 1940: Defending the Republic. New Haven: Yale University Press, 2015.